Subterranean deposits of coal contain substantial quantities of entrained methane gas limited in production in use of methane gas from coal deposits has occurred for many years. Substantial obstacles, however, have frustrated more extensive development and use of methane gas deposits in coal seams. The foremost problem in producing methane gas from coal seams is that while coal seams may extend over large areas of up to several thousand acres, the coal seams are fairly shallow in depth, varying from a few inches to several meters. Thus, while the coal seams are often relatively near the surface, vertical wells drilled into the coal deposits for obtaining methane gas can only drain a fairly small radius around the coal deposits. Further, coal deposits are not amendable to pressure fracturing and other methods often used for increasing methane gas production from rock formations. As a result, once the gas easily drained from a vertical well bore in a coal seam is produced, further production is limited in volume. Additionally, coal seams are often associated with subterranean water, which must be drained from the coal seam in order to produce the methane.
Horizontal drilling patterns have been tried in order to extend the amount of coal seams exposed to a drill bore for gas extraction. Such horizontal drilling techniques, however, require the use of a radiused well bore which presents difficulties in removing the entrained water from the coal seam. The most efficient method for pumping water from a subterranean well, a sucker rod pump, does not work well in horizontal or radiused bores.
A further problem for surface production of gas from coal seams is the difficulty presented by under balanced drilling conditions caused by the porousness of the coal seam. During both vertical and horizontal surface drilling operations, drilling fluid is used to remove cuttings from the well bore to the surface. The drilling fluid exerts a hydrostatic pressure on the formation which, if it exceeds the hydrostatic pressure of the formation, can result in a loss of drilling fluid into the formation. This results in entrainment of drilling finds in the formation, which tends to plug the pores, cracks, and fractures that are needed to produce the gas.
As a result of these difficulties in surface production of methane gas from coal deposits, the methane gas which must be removed from a coal seam prior to mining, has been removed from coal seams through the use of subterranean methods. While the use of subterranean methods allows water to be easily removed from a coal seam and eliminates under balanced drilling conditions, they can only access a limited amount of the coal seams exposed by current mining operations. Where longwall mining is practiced, for example, underground drilling rigs are used to drill horizontal holes from a panel currently being mined into an adjacent panel that will later be mined. The limitations of underground rigs limits the reach of such horizontal holes and thus the area that can be effectively drained. In addition, the degasification of a next panel during mining of a current panel limits the time for degasification. As a result, many horizontal bores must be drilled to remove the gas in a limited period of time. Furthermore, in conditions of high gas content or migration of gas through a coal seam, mining may need to be halted or delayed until a next panel can be adequately degasified. These production delays add to the expense associated with degasifying a coal seam.
Prior mining systems also generally require a fairly large and level surface area from which to work. As a result, prior mining systems and drilling technologies generally cannot be used in Appalachia or other hilly terrains. For example, in some areas the largest area of flat land may be a wide roadway. Thus, less effective methods must be used, leading to production delays that add to the expense associated with degasifying a coal seam.
Production of petroleum and other valuable materials from subterranean zones frequently results in the production of water and other by-products that must be managed in some way. Such by-product water may be relatively clean, or may contain large amounts of brine or other materials. These by-products are typically disposed of by simply pouring them at the surfaces or, if required by environmental regulations, hauling them off-site at great expense.
At any point in the drilling of a well bore its desired orientation may be vertical, horizontal or at any other orientation to achieve the positioning of the bore required by the incident application. Further, the incident application may require that the well bore remain within and/or aligned with one or more boundaries of a specific “target” geologic formation such as a stratum, seam or other delimited subterranean structure. In these cases, it is necessary to detect and measure the distance to the boundaries between the target formation and the adjacent formation(s) to allow guidance of the drilling process to keep the well bore within the target formation.
Well bores are typically formed by a drilling rig that rotates a drill string and thus a drill bit at the distal end of the drill string; or which rotates the drill string only to alter the direction of drilling, and the drill bit may in those cases be powered by, for example, a hydraulic or electric powered motor section located at or near the end of the drill string. The drill string may also include a bent section to facilitate steering and/or other rotation of the drill bit.
While the use of subterranean methods allows water to be easily removed from a coal seam and eliminates under-balanced drilling conditions, they can only access a limited amount of the coal seams exposed by current mining operations. Where longwall mining is practiced, for example, underground drilling rigs are used to drill horizontal holes from a panel currently being mined into an adjacent panel that will later be mined. The limitations of underground rigs limits the reach of such horizontal holes and thus the area that can be effectively drained. In addition, the degasification of a next panel during mining of a current panel limits the time for degasification. As a result, many horizontal bores must be drilled to remove the gas in a limited period of time. Furthermore, in conditions of high gas content or migration of gas through a coal seam, mining may need to be halted or delayed until a next panel can be adequately degasified. These production delays add to the expense associated with degasifying a coal seam.
Underreamers may be used to form an enlarged cavity in a well bore extending through a subterranean formation. The cavity may then be used to collect resources for transport to the surface, as a sump for the collection of well bore formation cuttings and the like or for other suitable subterranean exploration and resource production operations. Additionally, the cavity may be used in well bore drilling operations to provide an enlarged target for constructing multiple intersecting well bores.
One example of an underreamer includes a plurality of cutting blades pivotally coupled to a lower end of a drill pipe. Centrifugal forces caused by rotation of the drill pipe extends the cutting blades outwardly and diametrically opposed to each other. As the cutting blades extend outwardly, the centrifugal forces cause the cutting blades to contact the surrounding formation and cut through the formation. The drill pipe may be rotated until the cutting blades are disposed in a position substantially perpendicular to the drill pipe, at which time the drill pipe may be raised and/or lowered within the formation to form a cylindrical cavity within the formation.
Conventional underreamers, however, suffer several disadvantages. For example, the underreamer described above generally requires high rotational speeds to produce an adequate level of centrifugal force to cause the cutting blades to cut into the formation. An equipment failure occurring during high speed rotation of the above-described underreamer may cause serious harm to operators of the underreamer as well as damage and/or destruction of additional drilling equipment.
Additionally, density variations in the subsurface formation may cause each of the cutting blades to extend outwardly at different rates and/or different positions relative to the drill pipe. The varied positions of the cutting blades relative to the drill pipe may cause an out-of-balance condition of the underreamer, thereby creating undesired vibration and rotational characteristics during cavity formation, as well as an increased likelihood of equipment failure.
A common problem in producing methane gas from coal seams may be vertical separation of multiple thin layers of coal within a coal seam. Although coal seams may extend over large areas of up to several thousand acres, the depth of the multiple layers in the coal seam may vary from very shallow to very deep. Vertical wells drilled into the coal deposits for obtaining methane gas can only drain a fairly small radius of methane gas around the vertical well. Further, coal deposits are not amenable to pressure fracturing and other methods often used for increasing gas production from conventional rock formations. As a result, production of gas may be limited in volume. Additionally, coal seams are often associated with subterranean water, which must be drained from the coal seam in order to produce the methane.
One problem in producing methane gas from coal seams is that while coal seams may extend over large areas, up to several thousand acres, and may vary in depth from a few inches to many feet. Coal seams may also have a low permeability. Thus, vertical wells drilled into the coal deposits for obtaining methane gas can generally only drain a fairly small radius of methane gas in low and even medium permeability coal deposits. As a result, once gas in the vicinity of a vertical well bore is produced, further production from the coal seam through the vertical well is limited.
Another problem in producing methane gas from coal seams is subterranean water which must be drained from the coal seam in order to produce the methane. As water is removed from the coal seam, it may be replaced with recharge water flowing from other virgin areas of the coal seam and/or adjacent formations. This recharge of the coal seam extends the time required to drain the coal seam and thus prolongs the production time for entrained methane gas which may take five years, ten years, or even longer. When the area of the coal seam being drained is near a mine or other subterranean structure that reduces water and/or recharge water by itself draining water from the coal seam or in areas of high permeability, methane gas may be produced from the coal seam after a shorter period of water removal. For example, in Appalachia coal beds with a high permeability of ten to fifteen millidarcies have in four or five months been pumped down to the point where gas can be produced.
One problem of production of gas from coal seams may be the difficulty presented at times by over-balanced drilling conditions caused by low reservoir pressure and aggravated by the porosity of the coal seam. During both vertical and horizontal surface drilling operations, drilling fluid is used to remove cuttings from the well bore to the surface. The drilling fluid exerts a hydrostatic pressure on the formation which, when exceeding the pressure of the formation, can result in a loss of drilling fluid into the formation. This results in entrainment of drilling finds in the formation, which tends to plug the pores, cracks, and fractures that are needed to produce the gas.
Certain methods are available to drill in an under-balanced state. Using a gas such as nitrogen in the drilling fluid reduces the hydrostatic pressure, but other problems can occur as well, including increased difficulty in maintaining a desired pressure condition in the well system during drill string tripping and connecting operations.
Subterranean zones, such as coal seams, contain substantial quantities of entrained methane gas. Subterranean zones are also often associated with liquid, such as water, which must be drained from the zone in order to produce the methane. When removing such liquid, entrained coal fines and other fluids from the subterranean zone through pumping, methane gas may enter the pump inlet which reduces pump efficiency.
One problem of surface production of gas from coal seams may be the difficulty presented at times by over-balanced drilling conditions caused by the porosity of the coal seam. During both vertical and horizontal surface drilling operations, drilling fluid is used to remove cuttings from the well bore to the surface. The drilling fluid exerts a hydrostatic pressure on the formation which, if it exceeds the pressure of the formation, can result in a loss of drilling fluid into the formation. This results in entrainment of drilling finds in the formation, which tends to plug the pores, cracks, and fractures that are needed to produce the gas. Other problems include a difficulty in maintaining a desired pressure condition in the well system during drill string tripping and connecting operations.